1. Field of the Invention
The present invention relates to a method for converting a continuous tone image for approximately displaying a continuous tone image into a pseudo half tone image using an output apparatus which can only perform binary expression in units of pixels.
2. Description of the Related Art
As a conventional method for forming a half tone image, various methods have been proposed. For example, as described in Hirotsugu Yamada, Michihiko Mino, and Toshiyuki Sakai, "Binary Expression Method of Image Suitable for Intra-mesh Density Structure", Papers of the Society of Electronic Communications, 1985/4, Vol. J68-D, No. 4, an ordered dither method, a minimizer average error method, an intra-mesh pixel distribution method, and the like have been reported, and have been put into practical applications.
Upon formation of a half tone image, it is important to simultaneously improve the reproducibility of a density tone characteristic (gradation) and the resolution, and it is also important to suppress formation of a texture offensive to the eyes.
For example, assuming that a laser beam printer (LBP) for a relatively expensive OA equipment is used, in the ordered dither method, since the number of tones is determined by the matrix size, the matrix size must be increased to obtain a sufficient tone characteristic. However, when the matrix size is increased, black dots are arranged in large unit areas, resulting in a decrease in resolution.
When the minimizer average error method or a similar error diffusion method is used, a relatively good tone characteristic can be obtained. A summary of this method is described in, e.g., J. F. Javis, C. N. Judice, and W. H. Ninke, "A Survey of Techniques for the Display of Continuous Tone Pictures on Bilevel Displays", Computer Graphics and Image Processing 5 (1976), pp. 36-38. Many examples of, e.g., a generation method for a weighting matrix .alpha. used in this minimizer average error method have been proposed.
However, in the minimizer average error method or the error diffusion method, a unique stripe-pattern texture is formed, and is offensive to the eyes. Since this method is a method for minimizing an error of the overall image from an original image upon binarization with a predetermined threshold value, an error occurring at a given point sequentially propagates to pixels to be processed later. A texture formed in this manner is easily observed on a region where a change in tone level is small (the spatial frequency is low).
In this manner, when a density image is binarized and output using, e.g., an LBP for an OA equipment, and the ordered dither method is used, if the tone characteristic is to be improved, the resolution is impaired. Contrary to this, when the resolution is to be increased, the tone characteristic is degraded. Thus, it is difficult to simultaneously improve both the tone characteristic and the resolution. On the other hand, when the minimizer average error method is used, the tone characteristic can be improved, but a texture offensive to the eyes is undesirably formed on a region having a small change in tone level.
It is important for the LBP to prevent, e.g., a thin line from being disconnected or to prevent a short defect from being formed in a portion to be completely painted in black, as shown in FIG. 1, so as to clearly print, e.g., characters. For this reason, the LBP is designed to set an effective size of a black dot to be larger than a logical size of a pixel (determined by a minimum increment of raster scanning). More specifically, as shown in FIG. 2, a thin line having a one-dot width is drawn to be thicker than a logical one-dot width, so that a thin line can be prevented from being disconnected or a short defect can be prevented from being formed between raster scanning lines in a black painting region even under the influence of various noise components or variations in parameters.
In this manner, in the prior art, the intensity of each pixel is independently considered, i.e., it is "0" or "1" for an output binary image, and is a real number within a range between "0" and "1" for an input density image. For this reason, the fact that a black dot per pixel is printed to be larger in practice, and projects into an adjacent pixel region is ignored. Thus, in an LBP in which a degree of an increase in size of a black dot is large, an image to be obtained is unnecessarily darkened, or a dark portion (shadow) in an image undesirably has a batter.